1. Field of the Disclosure
The present disclosure is related to resistivity imaging of an earth formation penetrated by a borehole. In particular, the present disclosure is an apparatus and a method for imaging electrical properties of a formation at multiple resolution scales.
2. Background of the Art
Typically, borehole resistivity can be imaged using resistivity imaging devices conveyed downhole, for example, on a drill string. In one type of resistivity imaging device, such as is typically used in boreholes with conductive mud, one or more electrodes are used to emit current into a formation. A measurement of the amount of current emitted reflects a conductivity of the formation in front of the electrode. In such an imaging device, the resolution is proportional to the size of measure electrodes used. Large measure electrodes cover a large area and correspond to a low resolution scale, and small electrodes cover a small area and correspond to a high resolution scale. In another type of resistivity imaging, such as non-conductive mud imaging, typically a four-terminal device is used which includes an injection electrode and a return electrode for injecting current into a formation and at least two sensor electrodes to measure a potential gradient between the injection and return electrodes. For such a device, the image resolution is proportional to the spacing between the sensor electrodes. A large distance between sensor electrodes corresponds to a low resolution scale, and a short distance between sensor electrodes corresponds to a high resolution scale.
When designing resistivity imaging devices, two competing factors, image resolution and stability, must be considered. Stability is related to the signal-to-noise ratio of an image response. Generally, an electrode designed for high resolution suffers from a low signal-to-noise ratio and thus provides a less stable signal. Inversely, an electrode designed for low resolution has a high signal-to-noise ratio and thus provides a more stable signal. In order to design a resistivity imaging device, a balance is generally struck between stability concerns and resolution requirements.
Existing borehole resistivity imaging devices are designed to have a fixed resolution. However, in typical imaging, there is often a need to obtain resistivity images at a plurality of resolution scales. The present disclosure addresses this need.